Damping has been identified as an important principle for micromechanical devices to help reduce errors and speed settling while improving stability. Micromechanical devices can be fabricated with purely elastic supports, resulting in a classic spring-mass-damper system. In many microelectromechanical (MEMS) systems, the primary source of damping is air damping, which may be insufficient with systems with relatively large air gaps. Under-damped MEMS can lead to long settling times and make closed loop control challenging. The introduction of fluids can lead to over-damped MEMS.
Several known damping mechanisms exist such as fluid damping and mechanical friction. Prior art solutions to perfect damping in MEMS mirrors have used soft metals, such as gold, on the springs or the supports, while other solutions have used other soft or plastic materials on the springs. However, these prior art solutions typically cause hysteresis, an undesirable result bringing about uncertainty of the MEMS angular position.
Another prior art solution, described in “A Magnetically Damped Momentum Transfer Device to Reduce Chatter in a Micro-Mechanical Switch” 0-7803-5998-4/01 IEEE 2001, pages 265–268, a switch is described where the bounce after contact is reduced by using magnetic damping to reduce linear momentum while maintaining high contact forces. However, this prior art solution presents no viable solution for a rotating MEMS device.
In the control of a rotational MEMS device such as a mirror, it is desirable to have a fast response time and a fast settling time. The optimal response is generally achieved by reducing the mechanical Q factor such that it is close to one (1). Many MEMS devices do not have sufficient air damping to achieve a low Q, hence there is a need for a method to introduce additional damping without adding hysteresis or fluids.
Fluids can contaminate a MEMS device and add to the cost and complexity of packaging. Furthermore, variations in fluid properties can defocus an optical beam and cause insertion loss.
A need exists for a method and apparatus for magnetic damping as a means of reducing the Q of MEMS rotating mirrors without introducing hysteresis, narrow gaps or fluids.